Boundedness of quantum observables?

[Careful]

I don't understand this, ignoring C*-algebras, Spin is a bounded operator in the quantum mechanical theory of a single fermion. Some Hamiltonians are also bounded.

Spin is only a part of the total angular momentum and does not make physical sense as an operator by itself. It is the orbital angular momentum part which is unbounded. Further, you miss the entire point, one can construct plenty of bounded Hamiltonians but the natural variables from which they are constructed correspond to unbounded operators.

This is not a matter of bad variables, but it is a deep consequence of Lorentz covariance.

 

[dextercioby]

Could you explain this?

The physical states, as commonly axiomatized, are described by unit norm eigenvectors for time-independent Hamiltonians in the Schroedinger picture. But there are no unit norm eigenvectors for the Hamiltonian of a free non-relativistic particle moving freely in R^3. All of them lie in the Schwartz space S'(R^3, dx). So no physical states.

 

[DarMM]

The physical states, as commonly axiomatized, are described by unit norm eigenvectors for time-independent Hamiltonians in the Schroedinger picture. But there are no unit norm eigenvectors for the Hamiltonian of a free non-relativistic particle moving freely in R^3. All of them lie in the Schwartz space S'(R^3, dx). So no physical states.

That's what I thought you meant, but it is not correct. There are physical states, the whole Hilbert space \mathcal{L}^{2}(\mathbb{R}^{3}) is full of them. It's just that there are no states which are eigenstates of the Hamiltonian. There are still states which evolve unitarily under the Hamiltonian.

 

[Careful]

That's what I thought you meant, but it is not correct. There are physical states, the whole Hilbert space \mathcal{L}^{2}(\mathbb{R}^{3}) is full of them. It's just that there are no states which are eigenstates of the Hamiltonian. There are still states which evolve unitarily under the Hamiltonian.

You are falling over semantics and miss the point again. What people call physical states is a matter of agreement, it is a social construct without any deeper meaning. Even at this level, you constantly use the distributional states by means of the Fourier transform, still you wish to expell them to the margins. Let me turn the game around and you show us a Hilbert space construction which is adequate for QFT. I have given plenty of positive arguments against Hilbert space, so you show now a positive argument pro Hilbert space. Then, we will talk.

 

[dextercioby]

That's what I thought you meant, but it is not correct. There are physical states, the whole Hilbert space \mathcal{L}^{2}(\mathbb{R}^{3}) is full of them. It's just that there are no states which are eigenstates of the Hamiltonian. There are still states which evolve unitarily under the Hamiltonian.

Then what you say totally disagrees with what's axiomatized through the Schroedinger equation. The physical states are solutions of the SE. If the Hamiltonian is time-independent (true for the free particle in the Schroedinger picture), then the physical state must have the form

\Psi_{\mbox{physical}}(t) = \psi_E \exp{\frac{t p^2}{2mi\hbar}}

where \psi_E is an eigenfunction of the free-particle hamiltonian, a member of S'(R^3), so the whole physical state becomes a tempered distribution, thus contradicting the physical state postulate.

 

[A. Neumaier]

Then what you say totally disagrees with what's axiomatized through the Schroedinger equation. The physical states are solutions of the SE. If the Hamiltonian is time-independent (true for the free particle in the Schroedinger picture), then the physical state must have the form

\Psi_{\mbox{physical}}(t) = \psi_E \exp{\frac{t p^2}{2mi\hbar}}

where \psi_E is an eigenfunction of the free-particle hamiltonian, a member of S'(R^3), so the whole physical state becomes a tempered distribution, thus contradicting the physical state postulate.

DarMM is correct. The Schroedinger equation i hbar d ps(t)i/dt = H psi(t), which describes the dynamics of states in time has a unique solution for arbitrary initial conditions psi(0) in the Hilbert space.

Most states are not eigenstates - the latter are the very specialized solutions that lead to harmonically oscillating phases. If all states had to be eigenstates, nothing ever would happen in our universe, since eigenstates are stationary.

 

[Careful]

Most states are not eigenstates - the latter are the very specialized solutions that lead to harmonically oscillating phases. If all states had to be eigenstates, nothing ever would happen in our universe, since eigenstates are stationary.

That is YOUR problem, NOT mine. My theory deals with spacetime dependent local Hamiltonians and there is no issue with Haag's theorem and describing preferred states. Your comments here are negative, since you have no good way to define what a real observable is (you only utter that you do not believe it is connected to some global Hamiltonian because that would give you trouble indeed in the standard formalism; moreover you have an ''irrational'' wish to preserve Hilbert space). All you can do is spit into your hands, guess how a measurement apparatus ''thinks'' and pray to God you guessed right (at least at asymptotic infinity). You should not confuse the shortcomings of standard QFT with a valid comment regarding the interpretation of distributional states.

Moreover, I don't know if you are aware of this, but the correct states in the canonical quantization of gravity are stationary states with zero energy (and guess what? they are distributional, like the Kodama state). People call this the problem of time while I prefer ''the end of standard QFT''. :-)

 

[dextercioby]

DarMM is correct. The Schroedinger equation i hbar d ps(t)i/dt = H psi(t), which describes the dynamics of states in time has a unique solution for arbitrary initial conditions psi(0) in the Hilbert space..

(bolded by me). I think you're not really seeing my point, though it seems you're disputing it. What I emphasized by bold characters holds true iff the Hamiltonian (thus the dynamics), when properly defined and self-adjoint, has a purely discrete spectrum. Else, the whole equation (the psi, its derivative and the H) would no longer be living in a Hilbert space, but into extension of it obtained by the Gelfand - Maurin spectral theorem.

So your claim has only restricted validity and in no way refutes any of my arguments.

Most states are not eigenstates - the latter are the very specialized solutions that lead to harmonically oscillating phases. If all states had to be eigenstates, nothing ever would happen in our universe, since eigenstates are stationary.

Of course that the whole quantum dynamics and everything going around in the (quantum) world is driven by time-dependent Hamiltonians. I wasn't questioning that. In QM (and also in QFT for what is worth) for time-depending Hamiltonians the SE cannot be solved completely and one is forced to make approximations. If you say that <most states are not eigenstates>, I'll go further <most states are actually unknown, but we can very well do without them, because we've been smart enough and invented perturbation theory>.

 

[Careful]

(bolded by me). I think you're not really seeing my point, though it seems you're disputing it. What I emphasized by bold characters holds true iff the Hamiltonian (thus the dynamics), when properly defined and self-adjoint, has a purely discrete spectrum. Else, the whole equation (the psi, its derivative and the H) would no longer be living in a Hilbert space, but into extension of it by obtained by the Gelfand - Maurin spectral theorem.

Just to add that a discrete spectrum is not even enough. If the spectrum isn't bounded, domain issues will arise and not any initial conditions can be chosen. But I guess what Arnold wanted to say is that for those states in the domain of H (which of course remain in the domain of H under evolution), the Hilbert space picture actually works.
But what I want to do is pull this discussion away from some silly textbook prejudices people have to situations where it really matters. For example to QFT or quantum gravity: that is where these issues really show their theeth, not in standard QM.

 

[A. Neumaier]

The Schroedinger equation i hbar d ps(t)i/dt = H psi(t), which describes the dynamics of states in time has a unique solution for arbitrary initial conditions psi(0) in the Hilbert space.

What I emphasized by bold characters holds true iff the Hamiltonian (thus the dynamics), when properly defined and self-adjoint, has a purely discrete spectrum.

No. What you emphasized by bold characters holds iff the Hamiltonian is self-adjoint.

For in this case it is the infinitesimal generator of a 1-parameter group exp(itH), which is a bounded operator defined on the full Hilbert space, and psi(t)=exp(-it/hbar H)psi(0) is a well-defined solution of the Schroedinger equation for every psi(0) in the Hilbert space.

Thus the statement holds in _all_ well-defined and time-reversal invariant quantum theories.

Of course that the whole quantum dynamics and everything going around in the (quantum) world is driven by time-dependent Hamiltonians.

No. Most of quantum mechanics is done with time-independent Hamiltonians - for example the whole of quantum chemistry. If all states were eigenstates, chemical reactions would be impossible!

 

[A. Neumaier]

But what I want to do is pull this discussion away from some silly textbook prejudices people have to situations where it really matters. For example to QFT or quantum gravity: that is where these issues really show their theeth, not in standard QM.

It is against the rules of PF to hijack a thread whose goal is something different.

I started this thread and want to discuss here only that part of QM which has a rigorous mathematical foundation. This includes QFT only as far as it has been rigorously constructed, and excludes quantum gravity unless you can offer a construction of its dynamics - i.e., a rigorous proof of solvability of its dynamical equations.

If you want to discuss the boundedness question on the looser level of theoretical physics, you should open your own thread.

 

[dextercioby]

No. What you emphasized by bold characters holds iff the Hamiltonian is self-adjoint.

The self-adjointness of an operator with mixed spectrum is a tricky business, because the spectral equation for the operator no longer has solutions in the original Hilbert space. That's why i mentioned the restricting condition of purely discrete spectrum.

For in this case it is the infinitesimal generator of a 1-parameter group exp(itH), which is a bounded operator defined on the full Hilbert space, and psi(t)=exp(-it/hbar H)psi(0) is a well-defined solution of the Schroedinger equation for every psi(0) in the Hilbert space.

You'd be surprised to know (in case you didn't already) that the trick with the Stone's theorem and its converse can very well be carried to distribution spaces, simply because there's a spectral theorem for unitary operators as well (see Gelfand's book).

So if psi(0) doesn't live in the h-space because of the continuous spectrum, psi(t) defined the way you did won't live either.

No. Most of quantum mechanics is done with time-independent Hamiltonians - for example the whole of quantum chemistry. If all states were eigenstates, chemical reactions would be impossible!

The bolded one I agree with. The structure of matter (atomic physics, molecular physics, chemical bond) is indeed time-independent. But the last one implies that quantum chemistry doesn't cover chemical reactions. Then what fundamental theory does explain chemical reactions ?

 

[Careful]

No. What you emphasized by bold characters holds iff the Hamiltonian is self-adjoint.

For in this case it is the infinitesimal generator of a 1-parameter group exp(itH), which is a bounded operator defined on the full Hilbert space, and psi(t)=exp(-it/hbar H)psi(0) is a well-defined solution of the Schroedinger equation for every psi(0) in the Hilbert space.

Thus the statement holds in _all_ well-defined and time-reversal invariant quantum theories.

But not a single realistic theory satisfies your conditions! Even the harmonic oscillator Hamiltonian is not a self-adjoint operator on the whole of Hilbert space. What we mean with self-adjoint extensions of unbounded symmetric operators A is that there exists a B such that B* = B and A < B, but B is of course not everywhere defined. Therefore, Stone's theorem does not hold and one can safely relegate it to the trashbin. This is actually a key insight in my book which should not be taken lightly.

No. Most of quantum mechanics is done with time-independent Hamiltonians - for example the whole of quantum chemistry. If all states were eigenstates, chemical reactions would be impossible!

Sure, that's why it doesn't work. The linear time picture of QFT is not commensurable with the nonlinear time picture of GR.

You are too obsessed with simple theorems which allow for nice structures. There exist more general structures which are still within control, you know.

Careful

 

[Careful]

It is against the rules of PF to hijack a thread whose goal is something different.

I started this thread and want to discuss here only that part of QM which has a rigorous mathematical foundation. This includes QFT only as far as it has been rigorously constructed, and excludes quantum gravity unless you can offer a construction of its dynamics - i.e., a rigorous proof of solvability of its dynamical equations.

If you want to discuss the boundedness question on the looser level of theoretical physics, you should open your own thread.

There is no loose level of unbouded operators! Nothing of QFT nicely fits within the limited mathematical tools you are using; even free QFT requires tools which go beyond the theorems you are quoting and this is certainly the case for interacting QFT. If you have a rigorous construction of interacting QFT within the C* language I would love to see it. It would be an (impossible) breakthrough, say for QED ? Moreover, even QED is not rigorously solved yet, why would this then be the case for quantum gravity?

But on the other hand if you wish to study/discuss things which do not appear in nature, then I will withdraw from this thread. I actually thought that you still were a bit interested in that... since after all this is still PHYSICS forums.

 

[A. Neumaier]

The self-adjointness of an operator with mixed spectrum is a tricky business, because the spectral equation for the operator no longer has solutions in the original Hilbert space. That's why i mentioned the restricting condition of purely discrete spectrum.

But my statement is nevertheless correct (Hille-Yosida theorem). It has nothing to do with distributions or the existence of eigenvectors.

The bolded one I agree with. The structure of matter (atomic physics, molecular physics, chemical bond) is indeed time-independent.

... whereas you were saying in #68 that ''the whole quantum dynamics and everything going around in the (quantum) world is driven by time-dependent Hamiltonians.''

Please be consistent, else it is frustrating to discuss with you.

But the last one implies that quantum chemistry doesn't cover chemical reactions.

No. Quantum chemistry does cover chemical reactions, and my statement assumes that.

But quantum chemistry would not cover chemical reactions if your interpretation that all states must be eigenstates of the Hamiltonian were valid.

 

[A. Neumaier]

But on the other hand if you wish to study/discuss things which do not appear in nature, then I will withdraw from this thread.

Yes please. After all, there is a separate forum for quantum gravity.

By the way, most of what appears in nature (with exception only of the biggest and the tiniest) is described by quantum chemistry, which exclusively works in the Hilbert space setting.

I actually thought that you still were a bit interested in that... since after all this is still PHYSICS forums.

After all, mathematical physics and quantum physics on the Hilbert space level are also physics, though you think little of them.

 

[Careful]

Yes please. After all, there is a separate forum for quantum gravity.

By the way, most of what appears in nature (with exception only of the biggest and the tiniest) is described by quantum chemistry, which exclusively works in the Hilbert space setting.

Sure, because they use some glorified multiparticle formalism with bounded potentials from below and above. Nothing surprising about that... but ok I will withdraw myself

 

[dextercioby]

But my statement is nevertheless correct (Hille-Yosida theorem). It has nothing to do with distributions or the existence of eigenvectors.

Spoken like a true mathematician. (No irony here). I get your point, hopefully you've gotten mine, even though we may probably still disagree. :P

... whereas you were saying in #68 that ''the whole quantum dynamics and everything going around in the (quantum) world is driven by time-dependent Hamiltonians.''

Please be consistent, else it is frustrating to discuss with you.

Yes, you're kind of right with the lack of consistency, though someone putting both my statements one to the other would still understand that I'm making a difference between the quantum statics (time-independent Hamiltonians) and quantum dynamics (time-dependent ones).

No. Quantum chemistry does cover chemical reactions, and my statement assumes that.

But quantum chemistry would not cover chemical reactions if your interpretation that all states must be eigenstates of the Hamiltonian were valid.

My interpretation is that all physical states of systems with time-independent hamiltonians must be eigenstates (either in HS space or in one of its possible extensions) of the hamiltonian. .

With this part I'm pretty sure of being consistent throughout the thread.

What I reiterated is substantially different than what you made of my statements (and which is bolded in the quote).

 

[A. Neumaier]

someone putting both my statements one to the other would still understand that I'm making a difference between the quantum statics (time-independent Hamiltonians) and quantum dynamics (time-dependent ones).

But this is _very_ different from how the terms are used by everyone else.

Time-independent Hamiltonians describe both the static aspects (equilibrium) and the dynamic aspects (motion) of a quantum system.

My interpretation is that all physical states of systems with time-independent hamiltonians must be eigenstates (either in HS space or in one of its possible extensions) of the hamiltonian. .

Increasing the size of your statements doesn't make them less invalid.

Probably you meant: ''Stationary states of systems with time-independent Hamiltonians must be normalized eigenstates of the Hamiltonian.'' This is a correct statement, but it is about a very small subset of states, namely only the stationary ones.

 

[strangerep]

I started this thread and want to discuss here only that part of QM which has a rigorous mathematical foundation. [...]

Re-reading your original post in this thread, it's still rather vague (to me anyway)
what issue/question you intend for this thread. It seems to be wandering all over
the place.

Now that the disruptive element has left the room, would you perhaps re-state
your focus/question of this thread more precisely (assuming further discussion
is still desired) ?

 

[A. Neumaier]

Re-reading your original post in this thread, it's still rather vague (to me anyway) what issue/question you intend for this thread. It seems to be wandering all over the place.

would you perhaps re-state your focus/question of this thread more precisely (assuming further discussion is still desired) ?

It is difficult to keep a thread focused...

I took partially inconsistent comments from DarMM about unbounded observables in the C^* algebra approach to rigorous field theory as my starting point.

The intended goal was to discuss the limitations of C^* algebras in this regard, and what the possible alternatives are.

 

[A. Neumaier]

Apparently there's some work in the field of <algebras of unbounded operators> as this review article (and the quoted bibliography) shows:
http://arxiv.org/abs/0903.5446

Thanks. This is a nice paper that I didn't know before. I need to read it more carefully.

Well, on more careful reading I found it a bit disappointing. It sacrifices the product of unbounded operators completely!

But I want a concept that covers the algebra of differential operators on Schwartz space, which is the right space on which the physical observables for QM of one degree of freedom act. Here the product is always well-defined.

 

[Careful]

The intended goal was to discuss the limitations of C^* algebras in this regard, and what the possible alternatives are.

In that case, your comments regarding my contribution are not what they seemed at first sight.

 

[A. Neumaier]

In that case, your comments regarding my contribution are not what they seemed at first sight.

You completely removed the basis of the discussion, dropping the existence of a definite inner product, referring to what is needed for quantum gravity, so that nothing is left but speculation. A discussion can lead nowhere when there is no common ground on which the participants agree.

I want to keep _all_ structure that theoretical physicists use when discussing ordinary quantum mechanics - the definite inner product, the unitarity of exp(iA) for the traditional observables, the unbounded spectrum of the Hamiltonian, but to drop the shackles of C^*-algebra, which was imposed for mathematical, not physical considerations.

This is quite different from what you propose - to drop most of the structure that gives sensible restrictions to QM, and allows the application of powerful mathematics. This almost killed the purpose of the thread - so I protested when you announced that it is ineed your goal to move the topic away form where it was.

Thus the two things can hardly be discussed in a single thread, unless all connections to my interests in this thread are lost. If you open a new thread about observable in indefinite spaces (or whatever), we can discuss your interests there.

 

[Careful]

You completely removed the basis of the discussion, dropping the existence of a definite inner product, referring to what is needed for quantum gravity, so that nothing is left but speculation.

No speculation, operators in Krein space have been rigorously studied as well as spectral decompositions and so on. It is just much less known obviously.

I want to keep _all_ structure that theoretical physicists use when discussing ordinary quantum mechanics - the definite inner product, the unitarity of exp(iA) for the traditional observables, the unbounded spectrum of the Hamiltonian, but to drop the shackles of C^*-algebra, which was imposed for mathematical, not physical considerations.

That are indeed very limited goals (which do not even suit QFT). However, this was not clear from what you wrote before, I am not a mind reader you know.

 

[A. Neumaier]

I want to keep _all_ structure that theoretical physicists use when discussing ordinary quantum mechanics - the definite inner product, the unitarity of exp(iA) for the traditional observables, the unbounded spectrum of the Hamiltonian, but to drop the shackles of C^*-algebra, which was imposed for mathematical, not physical considerations.

That are indeed very limited goals (which do not even suit QFT).

They suit _all_ QFTs whose existence is currently known, and they were necessary for proving their existence.

The main reason why I want to keep these restrictions is precisely because I want to understand the most interesting open case, QED, from a rigorous point of view.

Dropping structure that is present would only rob one of mathematical tools, and thus make the goal - the rigorous construction of QED - even harder than necessary.

 

[Careful]

They suit _all_ QFTs whose existence is currently known, and they were necessary for proving their existence.

You mean, all these which are not realized in nature

The main reason why I want to keep these restrictions is precisely because I want to understand the most interesting open case, QED, from a rigorous point of view.

I explained you 20 times why you will never succeed in constructing a physical theory with those limited tools, but I am afraid that we are in a circle where I offer evidence which has not been fully worked out yet and where you offer nothing at all so far.

Dropping structure that is present would only rob one of mathematical tools, and thus make the goal - the rigorous construction of QED - even harder than necessary.

Again, entirely false... the mathematical tools roughly stay the same. The computations just become a bit more elaborate but that was to be expected, no?

 

[A. Neumaier]

The main reason why I want to keep these restrictions is precisely because I want to understand the most interesting open case, QED, from a rigorous point of view.

I explained you 20 times why you will never succeed in constructing a physical theory with those limited tools, but I am afraid that we are in a circle where I offer evidence which has not been fully worked out yet and where you offer nothing at all so far.

Nothing in your book or your discussions here on PF has anything to do with a rigorous construction of QED. Nothing in your book carries even the slightest tint of rigor. Therefore, repeating your reasons another 20 times will not convince me of their relevance.

The evidence about QED from a rigorous point of view - ''which has not been fully worked out yet'' - is as empty as your promise to stay out of this thread. Therefore this will be my final reply to you here. What you say has nothing to do with the topic under discussion.

 

[Careful]

Nothing in your book or your discussions here on PF has anything to do with a rigorous construction of QED.

Sure, nothing which has been done so far has anything to do with that. Moreover, saying that nothing in my book is rigorous is a tremendous lie, everything which is written out there isn't less rigorous than standard textbook QM or general relativity on the level of Robert Wald. But I know, you have never understood the rigor of unbouded operators.

The evidence about QED from a rigorous point of view - ''which has not been fully worked out yet'' - is as empty as your promise to stay out of this thread.

Likewise is your babbling about rigorous techniques for quantum physics.

Therefore this will be my final reply to you here. What you say has nothing to do with the topic under discussion.

Indeed, there is no topic of physical relevance. Let us fight elsewere, will we?

 

[dextercioby]

Up until this dispute between prof Neumaier and Careful, the thread makes a useful reading though.

I hope all parties agree that there's no 100% mathematically rigorous theory of quantum mechanics and quantum field theory in flat 4 Minkowski space-time and that work can still be done to achieving it, of course, if somebody is still interested in it and has not migrated towards strings and quantum gravity.

I think your dispute comes from the fact that there seems to be almost a void intersection between prof. Neumaier;s intentions/expectations and Careful's work part of which is published on arxiv.